The magnetic circuits in electric generators usually comprise a laminated core, e.g. of sheet steel with a welded construction. To provide ventilation and cooling the core is often divided into stacks with radial and/or axial ventilation ducts. For larger machines the laminations are punched out in segments which are attached to the frame of the machine, the laminated core being held together by pressure fingers and pressure rings. The winding of the magnetic circuit is disposed in slots in the core, the slots generally having a cross section in the shape of a rectangle or trapezium.
In multi-phase electric generators the windings are made as either single or double layer windings. With single layer windings there is only one coil side per slot, whereas with double layer windings there are two coil sides per slot. By coil side is meant one or more conductors combined vertically or horizontally and provided with a common coil insulation, i.e. an insulation designed to withstand the rated voltage of the generator to earth.
Double-layer windings are generally made as diamond windings whereas single layer windings in the present context can be made as diamond or flat windings. Only one (possibly two) coil width exists in diamond windings whereas flat windings are made as concentric windings, i.e. with widely varying coil width. By coil width is meant the distance in arc dimension between two coil sides pertaining to the same coil.
Normally all large machines are made with double-layer winding and coils of the same size. Each coil is placed with one side in one layer and the other side in the other layer. This means that all coils cross each other in the coil end. If there are more than two layers these crossings complicate the winding work and the coil end is less satisfactory.
It is considered that coils for rotating generators can be manufactured with good results within a voltage range of 3-20 kV.
It is also generally known that connection of a synchronous machine to a power network must be via a Δ/Y-connected or step-up transformer, since the voltage of the power network is generally higher than the voltage it has hitherto been able to achieve with the electric machine. Thus this transformer and the synchronous machine constitute integrated parts of a plant. The transformer entails an extra cost and also has the drawback that the total efficiency of the system is reduced. If, therefore, it were possible to manufacture electric generators for considerably higher voltages, the step-up transformer could be eliminated.
Although the dominant known technology for supplying current from a generator to a high-voltage network, a concept which in the present application applies to the level of 20 kV and upwards, preferably higher than 36 kV, is for a transformer to be inserted between the generator and the power network, it is already known to attempt to eliminate the transformer and generate the high voltage directly out to the power network at its voltage level. Such generators are described, for instance, in U.S. Pat. Nos. 4,429,244, 4,164,672 and 3,743,867.
However, the machine designs according to the above publications do not permit optimal utilization of the electromagnetic material in the stator.